somewhat drastic drop in fuel economy

[SuperHO]

can someone possibly explain how, with absolutely no change in the vehicle or driving habits, my fuel economy went from 28mpg average to 23mpg average over the course of one tank of gas, then staying there consistently for the subsequent 6 tanks, despite my best efforts?

 

[92MTX]

O2's getting lazy... Mebbe..
Mine always seem to go bad slowly, then die completely..
Might be changing the gas blend where you live..
My mileage varies a good bit with the Summer/Winter changeover.

 

[SuperHO]

i thought about that, too...but according to the gas station i frequent, they haven't made the switch yet. if the O2s were headin out, wouldn't i get a code?

 

[rubydist]

Eventually, but generally as the o2 sensors begin to croak they see the mixture as too lean, so they bump the fuel trims up to richen up the mixture. The CEL won't come on until the fuel trims get to the limit, which in most Ford tunes is +/- 25%. If the o2 is wrong, of course, this results in a mixture that is too rich and consequently your mileage will go down. If they haven't been changed for a long-ish time, I'd go ahead and put in new ones.

 

[CademiaX]

I had this problem a while back. Swapped out the O2's and the gas mileage returned to normal.